1. Field of the Invention
This invention relates to motion picture encoders employing motion picture compressive encoding techniques. This application is based on patent application No. Hei 9-4571 filed in Japan, the content of which is incorporated herein by reference.
2. Prior Art
As the conventional compressive encoding techniques of motion picture signals employed in the motion picture encoders, there are provided international standards called "MPEG-1 (ISO/IEC IS 11172-2)" and "MPEG-2 (ISO/IEC IS 13818-2, ITU-T H.262 recommendation)". Herein, "MPEG" stands for "Motion Picture Experts Group"; "ISO" stands for "International Organization for Standardization"; "ITU" stands for "International Telecommunication Union". Both of the standards employ the motion compensation in the forward and backward directions as the motion compensation predictive encoding method.
FIG. 7 shows a conception for the motion compensation of screen images in the forward and backward directions. Herein, a screen image (or picture on the screen) I0 corresponds to an "intra encoding picture" (hereinafter, referred to as "I picture"). This picture is subjected to encoding using only the information of the screen without using the motion compensation.
A screen image P3 corresponds to a forward motion compensation predictive encoding picture (hereinafter, referred to as "P picture"). This picture P3 is subjected to motion compensation predictive encoding using the I picture I0 as the reference picture. In the example of FIG. 7, a variable M is set a to represent a distance between the I picture and P picture.
After completion of the encoding of the I picture I0 and P picture P3, a picture B1 is subjected to encoding. Herein, the picture B1 corresponds to a bidirectional motion compensation predictive encoding picture (hereinafter, referred to as "B picture"). In the processing of the B picture B1, the present system performs forward motion compensation based on the I picture I0 which is a previous picture for the picture B1 with respect to time. In addition, the present system performs backward motion compensation based on the P picture P3 which is a subsequent picture for the picture B1 with respect to time. Next, a B picture B2 is subjected to encoding which is performed subsequently to the encoding of the B picture B1.
FIGS. 8 and 9 show time-related relationships between picture inputs and encoding processes. Specifically, the relationships of FIG. 8 are provided with regard to the case where only the forward motion compensation is performed, while the relationships of FIG. 9 are provided with regard to the case where both the forward motion compensation and backward motion compensation are performed. First, in the case of FIG. 8 where only the forward motion compensation is performed, the delay occurs due to differences in the manner of handling data in the picture input and encoding processes. That is, in the picture input, a supply of data is processed normally by using one line at a time as a unit. For the encoding processes, in the case of the MPEG-1 and MPEG-2, for example, a macro block constructed by 16.times.16 pixels is used as a unit of encoding. For this reason, it is necessary to provide the picture inputs of 16 lines in advance.
In the case of FIG. 9 which uses the backward motion compensation predictive encoding method, it is necessary to note that an input order of pictures is different from an order of encoding. Herein, a variable GOP is set to a value of "15" to represent an interval between I pictures while M is set to a value of "3". In the case where the present system uses the forward motion prediction based on the I picture and P picture only, it is necessary to provide a frame memory for retaining information of the reference picture. It is obvious from FIG. 9 that before performing the encoding process of the B picture, the present system waits for completion of the encoding process of the I picture or P picture which is the reference picture for the backward motion compensation prediction. In order to retain an original picture of the B picture during the above wait time, it is necessary to provide a number of frame memories representative of the value of M.
Meanwhile, there is provided a noise reducer of digital video signals, an example of which is disclosed by a book written by Mr. Takahiko Fukinuki and entitled "Multi-Dimensional Signal Processing of TV Pictures" (pp. 188-191), which is issued by the Daily Industrial Paper Company of Japan in 1988. According to this book, it is well known that the recursive filter functions effectively on the time axis. In such a recursive filter on the time axis, a frame memory having a storage of one screen image is required for the calculations of differences based on the previous pictures.
The conventional technology has already developed picture encoders having functions of motion compensation predictive encoding and noise reducing. This technology is described in a technical report written by Mr. Okubo and his members and entitled "Development of a Two Chip Real-Time MPEG2 SP@ML Video Encoder" in the technical-report collection C562 of the 1996 general meeting of the Institute of Electronics, Information and Communication Engineers of Japan, and in another technical report written by Mr. Kumaki and his members and entitled "A Chip Set for a Programmable Real-Time MPEG2 Video Encoder--A Chip Set Architecture and Controller LSI" in the technical report ICD95-102 (issued on August of 1995) of the Institute of Electronics, Information and Communication Engineers of Japan.
FIG. 10 shows a configuration of an encoder using multiple frame memories in accordance with the conventional system. The conventional system uses independent memory components respectively for the frame memory provided for the motion compensation prediction and the frame memory provided for the video signal pre-processing such as the noise reduction. As described above, the conventional system has a configuration which requires multiple frame memories each corresponding to a memory component having a large capacity. With such a configuration, it is difficult to reduce the size of the encoder system as well as of the cost of the encoder system.
A variety of proposals have been made with respect to reduction of the capacity of the frame memory, wherein one such proposal is disclosed in Japanese Patent Laid-Open Publication No. 61-52085. The configuration disclosed in the above paper is shown in FIG. 11. This configuration is designed to provide motion compensation which is actualized by only forward prediction based on the preceding picture. In FIG. 11, a noise elimination circuit corresponding to the block enclosed in the dotted line contains a memory-A 9104. Herein, signals which are delayed by a time of one picture or less are extracted from the memory-A 9104 and are subjected to motion vector detection. In short, one memory is used for both noise reduction and motion compensation. For this reason, the system of FIG. 11 has the advantage that the configuration thereof can be actualized using a relatively small memory capacity which is smaller than that of the conventional system and which provides memories for noise reduction and motion compensation respectively.
In the system of FIG. 11, however, the delay time of the memory-A is accurately less than the time of one picture. In order to make a sum of the delay of the system to coincide with the time of one picture, the noise elimination circuit requires a memory-B 9105. In addition, the system requires a frame memory 915 for local decoding signals, independently of the aforementioned memories. For this reason, the system of FIG. 11 is problematic in that the number of components for the frame memories is increased.